This invention relates generally to frictionless bearings, and more particularly to frictionless bearings where the anti-friction elements are prestressed. Ball bearings and roller bearings are examples of frictionless bearings, wherein the antifriction elements are balls and rollers, respectively. A prestressed anti-friction element means an anti-friction element that is stressed and compressed by the confining dimensions of the inner and outer races of the bearing before the bearing carries any load. For example, in an assembled pre-stressed roller bearing, the rollers (which may be hollow) are squeezed between the inner and outer races so that the rollers are elastically deformed to have an oval cross-sectional shape. This invention also relates to a method of assembling the present prestressed frictionless bearing, including without limitation pre-stressed ball bearings and roller bearings.
The prior art teaches roller bearings and ball bearings. A roller bearing consists of an annular inner race, a larger annular outer race, and cylindrical rollers that fit in between the inner and outer race. These parts are made to certain minimum tolerances, or error factors. They also must be designed with a certain amount of play or space between them so that they will easily fit together and to facilitate lubrication. Consequently, the assembled roller bearings have a certain amount of play or looseness in them, that results from the cumulative effect of manufacturing tolerances, and designed play. As a result, a shaft supported by a roller bearing has a certain freedom of radial movement within the roller bearing. This freedom of movement is a problem in high performance equipment, including machine tools, and turbines and causes early bearing failure in addition to reducing the accuracy of the equipment using the bearing.
Ball bearings are generally similar to roller bearings, except that cylindrical roller bearings are used in the latter case instead of spherical ball bearings.
French Patent No. 946,559 issued to Steffanini, published on June 8, 1949, U.S. Pat. No. 3,765,071 issued to Bowen on Oct. 16, 1973, U.S. Pat. No. 3,930,693 issued to Bowen on Jan. 6, 1976, and U.S. Pat. No. 4,232,914 issued to Bowen on Nov. 11, 1980, teach pre-stressed roller bearings and pre-stressed ball bearings. In these bearings when assembled and used, the inner races have a large enough diameter, and the outer races have a small enough diameter, so that when assembled the dimensions of the two races compress all the rollers. The rollers or balls do not fit between the races unless they are compressed into oval cross-sections. This prior art teaches a preferred embodiment of a roller bearing in which the rollers are hollow tubes.
The advantage of the prestressed roller on ball bearings is that a shaft supported by such a bearing is held with little or no freedom of radial movement even though the individual parts of the bearing are machined to conventional tolerances. This is because the rollers or balls are compressed in every direction around the inner race and the resulting compression and back pressure of the elastic rollers or balls act in an array around the inner race, in a flexible suspension system, together holding the inner race radially in place, although leaving the inner race free to spin. This system allows the flexible rollers to absorb deviations out of round of the rotating race, without an equivalent deviation of the support shaft. The resulting reduced radial play of the supported shaft can greatly extend the life of the bearing in high performance applications and increase the accuracy of the equipment using the bearing. Furthermore, these high precision results may be obtained with bearing parts made with parts that are made with conventional precision tolerances, and which therefore cost less while performing better than high precision parts in unstressed bearings.
These pre-stressed roller bearings promise to be a great advance for radial load bearings to supporting rotating shafts. Bearings of this type, specifically prestressed hollow roller bearings, have been made experimentally and tested with good results. These pre-stressed roller bearings, however, have not entered common use because they share a common problem. The common problem is that until the present invention, no design and method has been invented to assemble prestressed frictionless radial load bearings without damaging them.
The prior art's method has been to design and manufacture the bearing parts so that when assembled, the hollow roller bearing would be prestressed. Then the hollow roller bearings would be assembled into the outer race without the inner race, a process that does not require pre-stressing. At this point the inner race would be pressed and screwed into the assembly, thus compressing the hollow roller bearings. Of course, this axial movement of the inner race across the faces of the roller bearings under compressing pressure caused scoring of the bearing surfaces. This process was a difficult, special art, requiring special hydraulic presses, and even the best craftsman could not reliably produce an assembled prestressed roller bearing of known undamaged condition.
The problem of satisfactorily assembling prestressed roller or ball bearings, with ordinary skill and tools, without damaging the parts, is a problem of long standing without an answer until the present invention. At least since 1947, when Steffanini was granted the patent cited above, this has been an unsolved problem. Failure to solve this problem despite the best efforts of the bearing industry for at least 40 years, has been the primary reason that pre-stressed roller and ball bearings have not been commercialized and brought into general use. The present invention solves this problem for the first time and yields the new result that prestressed roller and ball bearings can now be in general use and be made into an economic success.
The process of heating selected parts to temporarily expand them, and thereby facilitate their permanent joining together with other parts, is well known and is sometimes called "sweating". As a process it is an alternative to welding, or to designing a part as a single part instead of two parts. For example, the barrels and bodies of guns are often made separately and then sweated together. That is, the gun body, with an annular female flange to receive the barrel, is heated and expands. The cold barrel, with a simple male end, is then inserted into the body of the gun. The gun body then cools and shrinks, gripping the barrel and fixing the barrel into the body. Practically speaking, the barrel and body are then permanently joined as one piece, without any possibility of movement of the barrel within the body. Sweating has always been used only to join two pieces into one piece without subsequent movement between the joined pieces. Sweating has never been used with bearings or prestressed bearings to stress them.
U.S. Pat. No. 2,449,944 issued to Johnson on Sept. 21, 1948, and U.S. Pat. No. 2,792,619 issued to Komm on May 21, 1957 both teach heating and then unheating selected parts of roller bearings merely to facilitate assembly. The Johnson patent teaches the sweating of a retainer ring onto a race, as a method of permanently attaching the ring to the race. The Komm patent teaches heating to expand a retainer lip or flange on an outer race, to permit its assembly over the inner race and rollers, and then cooling the retainer lip or flange to shrink it to retain in an assembled manner the entire device. But these patents deal only with conventional un-prestressed roller bearings, and the changes in temperature and dimension are not used to pre-stress the rollers, as is the case in the present invention.
It is well known that many materials, and most metals, expand when heated, and contract when cooled. However, this knowledge has never been used to design or assemble prestressed bearings, or to design and assemble parts of any kind that move in relation to each other after assembly.
A problem with assembled prestressed roller bearings in the past has been that the pressure between the rollers and their races has squeezed out the lubricants and protective coatings. Hence, when stored, oxidation has sometimes occurred at these pressured points of contact. Once the bearing is put into use, this contact oxidation has not been a problem because the motion of the bearing has uniformly distributed the anti-oxidant lubricant.